Cannot open minimserver

[simoncn]

I confirm that BubbleUPnP only runs on Android. This is a shame as it is an excellent control point and should do exactly what you need, including support for Chromecast Audio.

I haven't used Plex, so I am not familiar with how it works. A quick web search indicates that there is a Plex app for iOS, so this is presumably the control point (probably only working with the Plex server). The renderer (in UPnP terminology) might also be built into the Plex app, using Chromecast Audio as its audio player (I haven't used Chromecast Audio either).

Although I am talking about this using UPnP terminology, it could be that the various components of Plex don't use the UPnP protocols to talk to each other. The advantage of UPnP is that it is an open standard with many devices supporting it. The disadvantage is that doing a "mix and match" of different vendor products is likely to be more complex that using an integrated solution from a single vendor.